1. Field of the Invention
This invention relates to photographic lens system, and more particularly to a method of widening the angular field of view of a varifocal or mono-focal objective lens.
2. Description of Prior Art
To widen the angular field of view of a zoom lens, it is known to employ a wide converter lens as attached to the front of the zoom lens at the wide angle setting thereof. When the wide converter lens is afocal, the combined lens system is adjustable for focusing purposes within the same range as the zoom lens itself has. When it is not afocal, a change is resulted in the focusing range. In the latter connection, the focusing adjustment must be modified. As far as the widening of the angular field of view by use of the wide converter lens is concerned, therefore, it is required that the converter lens be afocal in itself, e.g. of the inverted Galileian type afocal converter lens. This requirement has conventionally been fulfiled by constructing the converter lens from a front concave lens group and a rear convex lens group satisfying the following relationship: EQU .vertline.f.sub.1 .vertline.+D=f.sub.2
wherein
f.sub.1 is the focal length of the concave lens group;
f.sub.2 is the focal length of the convex lens group; and
D is the interval between the principal points of the convex and concave lens groups.
The wide converter lens of such a construction has, however, the following problems. (i) The provision of the two lens groups of different sign power causes embodiment of a relatively complex lens structure which in turn makes the converter lens quite heavy and bulky. (ii) For facilitating a further increase in the field angle with decreasing angular magnification, it is necessary either to increase the principal point interval D, or to decrease the focal lengths f.sub.1 and f.sub.2 of the concave and convex lens groups with sacrificing the increase of the weight and bulk of the complete converter lens. Such problems become more serious as the field angle is increased. Accordingly, it has been difficult to increase the field angle of the zoom lens by use of the wide converter attachment lens based on the above mentioned relationship.